Remove dependency on spire about coulomb HOT 8 CLOSED

The goal is to avoid direct dependency on spire so that it does not block port to Scala 3. Having it be %Test is not a total avoidance, but it would allow continued testing in Scala 2, and testing in Scala 3 could just use native numeric types to start, plus in-tree Rational

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One option would be to implement a sort of MVP rational that is not necessarily a direct port of spire Rational. As used by coulomb core, the Rational is compile-time only in the implicit system, so less-than-maximum efficiency is lower impact.

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I guess that is an option. I'm trying to get spire to run in dotty but I don't have an ETA

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The three main things coulomb currently consumes from spire are:

1. Rational
2. ConvertableTo and ConvertableFrom
3. MultiplicativeGroup and friends

Rational would be easy to replace internally. MultiplicativeGroup and friends are being passed thru from typelevel algebra. I think the trickiest one to replace would be ConvertableTo and ConvertableFrom.

It would be feasible to define a coulomb typeclass for ConvertableTo and ConvertableFrom, and simply pass-thru the spire version whereever that was in scope.

My two main concerns with spire are that (1) it took a long time for spire to get onto scala-2.13, and I hope to not have it block coulomb for scala 3, and (2) spire defines an awful lot of stuff, and I'm only using a couple things.

Regardless of whether it is a hard dependency, I want coulomb to always integrate seamlessly with spire numeric types, for obvious reasons.

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side note: I'm also using shapeless for :: and HNil, which I'm getting transitively from spire but these types are purely implementation details, and they would be trivial to replace with my own version of :: and HNil internally

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cross-ref: over on my scala-3-POC I have an in-tree Rational and fractional exponents:

https://github.com/erikerlandson/poc-coulomb-scala3/blob/cbf290521792974b0f78767495a8caa09fdcc974/coulomb/src/coulomb/infra/meta.scala#L231

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my current design concept: coulomb-core will have no dependencies. It will come with coulomb.ops.standard and coulomb.ops.standard.truncating rules that cover operations, the new scala.Conversion and to / toUnit / toValue

There will be separate "tiers" for typelevel algebra, and then additionally spire numeric types.

see for example:
https://github.com/erikerlandson/poc-coulomb-scala3/blob/main/coulomb/src/coulomb/ops/standard.scala

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as of coulomb 0.6.0-M2 we have a separate coulomb-spire package, so I am going to close this

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